Electronic integrated circuits frequently need a power supply detection circuit to initialize the circuits for ensuring predictable operation after the power supply has reached a specified value. In the past, power supply reset circuits have used resistor and capacitor delay constants to create a delayed version of the power supply. The integrated circuit is fully powered by the time the delayed power supply signal reaches its final value. The initialization only occurs when the delayed power supply signal obtains a predetermined value. Therefore, the integrated circuits are fully operational by the time the initialization occurs. The length of the delay is varied by changing the values of the resistor and capacitor. However, a resistor-capacitor (RC) time delay initialization circuit is limited in its usefulness. The power supply may rise so slowly that large values of resistors and capacitors must be used. If implemented on the integrated circuit, the delay resistors and capacitors will cover a large amount of valuable substrate surface area. Another approach is to place the delay resistor and capacitor outside of the integrated circuit. This approach is also problematic because valuable integrated circuit pins must be used to access the delay resistors and capacitors. The delay resistors and capacitors will also use printed circuit board area and add to the number of components that must be placed on the printed circuit board. Problems can also arise when the power supply rises slower than the RC time constant. In this case, the circuit initialization may occur to early. For example, when the power supply is turned off, the RC time delay network must discharge before the power supply can be turned back on. If the power supply is turned back on too soon, the initialization may occur too early.
A power supply detection circuit which is physically on the integrated circuit, and which uses no resistors or capacitors but does use only the standard enhancement mode NMOS and PMOS transistors, would be advantageous because less integrated circuit substrate area would be used, and no integrated circuit pins would be used. U.S. Pat. No. 5,323,067 describes a solution that incorporates these advantages. However, this method does not allow for the integrated circuit to initialize when the power supply is cycled quickly. That is, if the power supply to the integrated circuit is rapidly turned from the state of "on" to the state of "off" and then back to the state of "on", there is no guarantee that initialization of the integrated circuit will occur. The method of this patent also requires an excessive amount of circuit elements which occupy valuable integrated circuit substrate surface area.
It is desirable that a power supply detection circuit system use the least amount of circuit elements so that the amount of integrated circuit substrate surface area is minimized. A power supply detection circuit should also reliably initialize the integrated circuit at a precise but adjustable power supply voltage level. It is desirable to be able to guarantee initialization of the integrated circuits when the power supply is cycled to ensure that the circuitry is properly initialized.
Thus, there is a need for a power supply detection circuit that uses a minimal amount of integrated circuit substrate surface area, dissipates a minimal amount of power and allows the power supply initialization detection voltage to be variable. The power supply initialization should also be reliable and predictable when the power supply is rapidly cycled over a short period of time.